Skype and MSN Data usage?

[Guest 91catalunya]

Hi guys, I was wondering how much data Skype uses? Not the voice calls as I am guessing this could be quite high but how much the instant messaging uses? Also do you know how much MSN uses? Will only use data while sending and reciving messages, or will it use data if just left running and logged in?

I only have the pathetic 250mb Orange internet package and dont want to go over it?

Thank you in advance :-)

Chris...